package java.util;
import java.io.IOException;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
/**
* 1）LinkedHashMap 按照键值对的插入顺序进行遍历，LinkedHashMap 底层通过一个双向链表来维护
* Entry 的顺序，重新插入已经存在的键，不会影响迭代顺序。
* 2）LinkedHashMap 的 collection 视图迭代器所需时间与映射的大小成正比，而 HashMap 迭代所需的时间
* 与其容量成正比。
* 3）LinkedHashMap 返回的迭代器都是快速失败的，如果从结构上对其进行修改，除非使用迭代器自身的
* remove 方法，否则迭代器将抛出 ConcurrentModificationException 异常。
*/
public class LinkedHashMap<K,V>
extends HashMap<K,V>
implements Map<K,V>
{
/**
* 带有前置节点和后置节点的 Entry
*/
static class Entry<K,V> extends HashMap.Node<K,V> {
Entry<K,V> before, after;
Entry(int hash, K key, V value, Node<K,V> next) {
super(hash, key, value, next);
}
}
private static final long serialVersionUID = 3801124242820219131L;
/**
* The head (eldest) of the doubly linked list.
* 双向链表的头结点
*/
transient LinkedHashMap.Entry<K,V> head;
/**
* The tail (youngest) of the doubly linked list.
* 双向链表的尾节点
*/
transient LinkedHashMap.Entry<K,V> tail;
/**
* The iteration ordering method for this linked hash map: {@code true}
* for access-order, {@code false} for insertion-order.
* 遍历时的访问顺序，true 表示访问越多越靠前，false 表示按照插入顺序进行遍历，
* LinkedHashMap 可以用于实现 LRU 缓存。
* @serial
*/
final boolean accessOrder;
/**
* 将节点添加到双向链表的尾部
* created by ZXD at 17 Jul 2018 T 20:32:02
* @param p
*/
private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {
// 暂存尾节点
LinkedHashMap.Entry<K,V> last = tail;
// 将尾节点设置为当前节点
tail = p;
if (last == null) // 尾节点为空
head = p; // 头结点设置为当前节点
else {
p.before = last; // 当前节点的前置节点设置为旧的尾节点
last.after = p; // 旧尾节点的后置节点设置为当前节点
}
}
// apply src's links to dst
private void transferLinks(LinkedHashMap.Entry<K,V> src,
LinkedHashMap.Entry<K,V> dst) {
LinkedHashMap.Entry<K,V> b = dst.before = src.before;
LinkedHashMap.Entry<K,V> a = dst.after = src.after;
if (b == null)
head = dst;
else
b.after = dst;
if (a == null)
tail = dst;
else
a.before = dst;
}
// overrides of HashMap hook methods
void reinitialize() {
super.reinitialize();
head = tail = null;
}
/**
* 创建新节点并将其链接到双向链表尾部
* created by ZXD at 21 Jul 2018 T 09:31:07
*/
Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
LinkedHashMap.Entry<K,V> p =
new LinkedHashMap.Entry<>(hash, key, value, e);
linkNodeLast(p);
return p;
}
/**
* 节点替换
* created by ZXD at 21 Jul 2018 T 09:32:21
*/
Node<K,V> replacementNode(Node<K,V> p, Node<K,V> next) {
LinkedHashMap.Entry<K,V> q = (LinkedHashMap.Entry<K,V>)p;
LinkedHashMap.Entry<K,V> t =
new LinkedHashMap.Entry<>(q.hash, q.key, q.value, next);
transferLinks(q, t);
return t;
}
TreeNode<K,V> newTreeNode(int hash, K key, V value, Node<K,V> next) {
TreeNode<K,V> p = new TreeNode<>(hash, key, value, next);
linkNodeLast(p);
return p;
}
TreeNode<K,V> replacementTreeNode(Node<K,V> p, Node<K,V> next) {
LinkedHashMap.Entry<K,V> q = (LinkedHashMap.Entry<K,V>)p;
TreeNode<K,V> t = new TreeNode<>(q.hash, q.key, q.value, next);
transferLinks(q, t);
return t;
}
void afterNodeRemoval(Node<K,V> e) { // unlink
// 记录被删除节点的前置节点和后置节点
LinkedHashMap.Entry<K,V> p =
(LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
// 将目标节点从链表中移除
p.before = p.after = null;
if (b == null) // 前置节点为 null，则更新头结点为后置节点
head = a;
else
b.after = a; // 将后置节点链接到前置节点的尾部
if (a == null) // 后置节点为 null，则更新尾节点为前置节点
tail = b;
else
a.before = b; // 将前置节点链接到后置节点的前面
}
void afterNodeInsertion(boolean evict) { // possibly remove eldest
LinkedHashMap.Entry<K,V> first;
if (evict && (first = head) != null && removeEldestEntry(first)) {
K key = first.key;
removeNode(hash(key), key, null, false, true);
}
}
void afterNodeAccess(Node<K,V> e) { // move node to last
LinkedHashMap.Entry<K,V> last;
// 如果 accessOrder 为 true，并且当前节点 e 不是尾节点，则将其移动到尾部
if (accessOrder && (last = tail) != e) {
LinkedHashMap.Entry<K,V> p =
(LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
p.after = null;
if (b == null)
head = a;
else
b.after = a;
if (a != null)
a.before = b;
else
last = b;
if (last == null)
head = p;
else {
p.before = last;
last.after = p;
}
tail = p;
++modCount;
}
}
void internalWriteEntries(java.io.ObjectOutputStream s) throws IOException {
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after) {
s.writeObject(e.key);
s.writeObject(e.value);
}
}
/**
* 根据指定的初始容量和加载因子创建，优先根据插入顺序进行遍历的 LinkedHashMap 实例
*/
public LinkedHashMap(int initialCapacity, float loadFactor) {
super(initialCapacity, loadFactor);
accessOrder = false;
}
/**
* 根据指定的初始容量和默认加载因子创建，优先根据插入顺序进行遍历的 LinkedHashMap 实例
*/
public LinkedHashMap(int initialCapacity) {
super(initialCapacity);
accessOrder = false;
}
/**
* 根据默认的初始容量和加载因子创建，优先根据插入顺序进行遍历的 LinkedHashMap 实例
*/
public LinkedHashMap() {
super();
accessOrder = false;
}
/**
* 基于目标 Map 创建，优先根据插入顺序进行遍历的 LinkedHashMap 实例
*/
public LinkedHashMap(Map<? extends K, ? extends V> m) {
super();
accessOrder = false;
putMapEntries(m, false);
}
/**
* 根据默认的初始容量、加载因子、访问顺序创建 LinkedHashMap 实例
*/
public LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder) {
super(initialCapacity, loadFactor);
this.accessOrder = accessOrder;
}
/**
* Returns {@code true} if this map maps one or more keys to the
* specified value.
* LinkedHashMap 中是否包含指定的值
*/
public boolean containsValue(Object value) {
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after) {
V v = e.value;
if (v == value || (value != null && value.equals(v)))
return true;
}
return false;
}
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
* 根据指定的键获取值，键不存在或值为 null 的情况下都返回 null，
* 此方法无法区分键不存在还是值为 null，键的存在性需要通过 containsKey 进行判断。
*/
public V get(Object key) {
Node<K,V> e;
if ((e = getNode(hash(key), key)) == null)
return null;
// 目标节点存在，并且 accessOrder 为 true，则将该节点移动到 LinkedHashMap 尾部
if (accessOrder)
afterNodeAccess(e);
return e.value;
}
/**
* 节点不存在则返回默认值
*/
public V getOrDefault(Object key, V defaultValue) {
Node<K,V> e;
if ((e = getNode(hash(key), key)) == null)
return defaultValue;
if (accessOrder)
afterNodeAccess(e);
return e.value;
}
/**
* 清空 LinkedHashMap
*/
public void clear() {
super.clear();
head = tail = null;
}
/**
* Returns {@code true} if this map should remove its eldest entry.
* This method is invoked by {@code put} and {@code putAll} after
* inserting a new entry into the map. It provides the implementor
* with the opportunity to remove the eldest entry each time a new one
* is added. This is useful if the map represents a cache: it allows
* the map to reduce memory consumption by deleting stale entries.
*
* <p>Sample use: this override will allow the map to grow up to 100
* entries and then delete the eldest entry each time a new entry is
* added, maintaining a steady state of 100 entries.
* <pre>
* private static final int MAX_ENTRIES = 100;
*
* protected boolean removeEldestEntry(Map.Entry eldest) {
* return size() > MAX_ENTRIES;
* }
* </pre>
* 在调用 put 和 putAll 方法插入一个新的条目时，如果该方法返回值为 true，则 LinkedHashMap
* 会移除最先插入的键值对，该方法适合用于实现 FIFO 的 JVM 缓存。
*/
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
return false;
}
/**
* 返回 LinkedHashMap 的键集合视图
*/
public Set<K> keySet() {
Set<K> ks = keySet;
if (ks == null) {
ks = new LinkedKeySet();
keySet = ks;
}
return ks;
}
final class LinkedKeySet extends AbstractSet<K> {
public final int size() { return size; }
public final void clear() { LinkedHashMap.this.clear(); }
public final Iterator<K> iterator() {
return new LinkedKeyIterator();
}
public final boolean contains(Object o) { return containsKey(o); }
public final boolean remove(Object key) {
return removeNode(hash(key), key, null, false, true) != null;
}
public final Spliterator<K> spliterator() {
return Spliterators.spliterator(this, Spliterator.SIZED |
Spliterator.ORDERED |
Spliterator.DISTINCT);
}
public final void forEach(Consumer<? super K> action) {
if (action == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
action.accept(e.key);
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
/**
* 返回 LinkedHashMap 的值集合视图
*/
public Collection<V> values() {
Collection<V> vs = values;
if (vs == null) {
vs = new LinkedValues();
values = vs;
}
return vs;
}
final class LinkedValues extends AbstractCollection<V> {
public final int size() { return size; }
public final void clear() { LinkedHashMap.this.clear(); }
public final Iterator<V> iterator() {
return new LinkedValueIterator();
}
public final boolean contains(Object o) { return containsValue(o); }
public final Spliterator<V> spliterator() {
return Spliterators.spliterator(this, Spliterator.SIZED |
Spliterator.ORDERED);
}
public final void forEach(Consumer<? super V> action) {
if (action == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
action.accept(e.value);
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
/**
* 返回 LinkedHashMap 的键值对集合视图
*/
public Set<Map.Entry<K,V>> entrySet() {
Set<Map.Entry<K,V>> es;
return (es = entrySet) == null ? (entrySet = new LinkedEntrySet()) : es;
}
final class LinkedEntrySet extends AbstractSet<Map.Entry<K,V>> {
public final int size() { return size; }
public final void clear() { LinkedHashMap.this.clear(); }
public final Iterator<Map.Entry<K,V>> iterator() {
return new LinkedEntryIterator();
}
public final boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<?,?> e = (Map.Entry<?,?>) o;
Object key = e.getKey();
Node<K,V> candidate = getNode(hash(key), key);
return candidate != null && candidate.equals(e);
}
public final boolean remove(Object o) {
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>) o;
Object key = e.getKey();
Object value = e.getValue();
return removeNode(hash(key), key, value, true, true) != null;
}
return false;
}
public final Spliterator<Map.Entry<K,V>> spliterator() {
return Spliterators.spliterator(this, Spliterator.SIZED |
Spliterator.ORDERED |
Spliterator.DISTINCT);
}
public final void forEach(Consumer<? super Map.Entry<K,V>> action) {
if (action == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
action.accept(e);
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
// Map overrides
public void forEach(BiConsumer<? super K, ? super V> action) {
if (action == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
action.accept(e.key, e.value);
if (modCount != mc)
throw new ConcurrentModificationException();
}
public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
if (function == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
e.value = function.apply(e.key, e.value);
if (modCount != mc)
throw new ConcurrentModificationException();
}
// Iterators
abstract class LinkedHashIterator {
LinkedHashMap.Entry<K,V> next;
LinkedHashMap.Entry<K,V> current;
int expectedModCount;
LinkedHashIterator() {
next = head;
expectedModCount = modCount;
current = null;
}
public final boolean hasNext() {
return next != null;
}
final LinkedHashMap.Entry<K,V> nextNode() {
LinkedHashMap.Entry<K,V> e = next;
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (e == null)
throw new NoSuchElementException();
current = e;
next = e.after;
return e;
}
public final void remove() {
Node<K,V> p = current;
if (p == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
current = null;
removeNode(p.hash, p.key, null, false, false);
expectedModCount = modCount;
}
}
final class LinkedKeyIterator extends LinkedHashIterator
implements Iterator<K> {
public final K next() { return nextNode().getKey(); }
}
final class LinkedValueIterator extends LinkedHashIterator
implements Iterator<V> {
public final V next() { return nextNode().value; }
}
final class LinkedEntryIterator extends LinkedHashIterator
implements Iterator<Map.Entry<K,V>> {
public final Map.Entry<K,V> next() { return nextNode(); }
}
}
LinkedHashMap 源码分析
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